In many cities, current concentrations of particulate matter exceed the limit values established by the European Union. In the first 3 months of 2005, for example, the daily limit value was exceeded on more than 30 days in several German cities. View image

As early photosynthesis began to raise atmospheric oxygen levels about 2.7 billion years ago, iron precipitated out of seawater, creating banded iron formations like this specimen from Ontario, Canada. View image

Water flows continuously into the Berkeley Pit from surface runoff and groundwater seepage. To prevent the pit from contaminating surface aquifers, the cleanup plan calls for pumping and treating water from the pit to keep the water surface at a safe level. View image

Zebra mussels can survive in many aquatic environments and breed prolifically. Originally found in Russia, they have spread throughout the Great Lakes and the Mississippi, Ohio, Tennessee, and Hudson river basins. View image

Commercial fishing is one of the most dangerous occupations in the United States, but fishermen can make tens of thousands of dollars for one to three months' work. These rates represent a risk premium that is required to attract workers. View image

Large hog farms may house 10,000 or more hogs indoors for their entire life spans. On many U.S. farms, breeding sows are kept for weeks at a time in individual crates too narrow for the animals to turn around. View image

Along with wheat, maize (corn), and potatoes, rice is one of the world's most important staple foods. These foods contain widely varying levels of many important micronutrients. Golden rice is designed to eliminate one deficiency by producing vitamin A in its grains. View image

On a feedlot, young beef cattle are grown to market weight on a diet that consists primarily of grain. Cattle that are fed grain reach market size at about 14 months of age, compared to 18 to 24 months for grass-fed beef, but raising cattle on grain is much more energy-intensive. View image

Kimberella was originally thought to be a type of jellyfish, but scientists now believe that it had rigid parts (probably a hard, shell-like covering) which formed the deep depressions in these fossils. View image

The Framingham Heart Study was designed to identify general causes of heart disease and stroke. Researchers have published more than 1,000 scientific papers on risk factors for cardiovascular disease based on Framingham data. View image

A small settlement just north of the border between Egypt and Sudan taps an underground aquifer to support farming in the desert. Each dark spot is an irrigated crop circle about 1 kilometer across. View image

Invasive salt cedar along the Forgotten River segment of the Rio Grande in Hudspeth County, Texas and Chihuahua State, Mexico. Yellow areas indicate high probability of salt cedar infestation. View image

Archaeopteryx had some features not seen in today's birds, such as a long bony tail and a full set of teeth. However, it also had feathers, wings, and reduced fingers, which are characteristic of modern birds. View image

Excess sediments can clog river channels and damage wildlife habitat. If sediments are contaminated with pesticides or industrial chemicals, they can also reduce water quality and introduce hazardous substances into the food chain. View image

Some U.S. states and electric power companies offer rebates for installing home solar systems, which reduce peak demand for electricity and help avoid the need to build new generating facilities. View image

Stromatolites are uncommon today because burrowing and grazing organisms destabilize marine sediments and consume the microbial mats produced by cyanobacteria. Exceptions occur in hyper-saline environments, like Shark Bay, where few organisms can survive. View image

The Willamette Meteorite is probably a fragment from the core of an ancient planet that broke up as it orbited the sun. Its structure suggests that it sustained at least two high-energy impacts in space before it fell to Earth's surface and weathered further. View image

Some researchers believe that fuel cells could eventually
replace internal combustion engines. But converting to
hydrogen for transportation would require the
development of a new fleet of fuel-cell vehicles,
efficient methods for producing hydrogen on a large
scale, and a hydrogen storage and distribution system. View image

Chemical plants near the Palos Verdes Peninsula south of Los Angeles discharged contaminated sewage until the early 1970s. State agencies closed the local fishery for white croaker in 1990 due to health risks from eating fish contaminated with PCBs and pesticides. View image